e14228 Background: Kynurenine production by indoleamine 2,3-dioxygenase (IDO) is critical for tumor immune suppression through effector T cell anergy and regulatory T cell proliferation. This has led to the rapid development of IDO inhibitors for cancer immunotherapy. However, results from recent clinical trials have been disappointing and this is partly due to pathway redundancy. Tryptophan 2,3-dioxygenase (TDO), another important enzyme of the kynurenine pathway, plays a compensatory role in the absence of IDO activity. Therefore, we developed a dual inhibitor of IDO and TDO to achieve maximal inhibition of the kynurenine pathway and alleviate tumor immune suppression. Methods: Small-molecule inhibitors of IDO and TDO were synthesized and evaluated using in vitro cell-based assays. Pharmacokinetic and pharmacodynamic profiles were assessed for these inhibitors. Tumor-bearing mice were treated with CMG017 per os, either alone or in combination with immune checkpoint inhibitors (ICIs). The tumor microenvironment (TME) was assessed through histological, flow-cytometric, and Nanostring immune profiling analyses. Results: CMG017 suppressed kynurenine production more effectively than inhibitors targeting either IDO or TDO alone, in various human and murine cancer cell lines. Single administration of CMG017 showed favorable pharmacokinetic profiles compared with an IDO1 selective inhibitor. Repeated once-daily per os administration of CMG017 decreased kynurenine concentration in both tumors and plasma of tumor-bearing mice and delayed tumor growth without significant toxicity. CMG017 induced dramatic changes in immune-related genes in TME and enhanced intratumoral infiltration of CD8+ effector T cells. The anti-tumor activity of CMG017 was almost negated when T cells were depleted, indicating the importance of adaptive immunity for the in vivo efficacy of CMG017. Of note, combination immunotherapy of CMG017 with ICIs (anti-PD-1 and anti-CTLA-4) led to durable tumor regression and long-term overall survival. Mice with complete tumor regression were immune to tumor re-challenge, indicating the establishment of immunological memory. Conclusions: CMG017, a dual inhibitor of IDO and TDO, potently suppressed the kynurenine pathway and showed promising anti-cancer efficacy, with favorable pharmacologic profiles.